An imaging apparatus such as video camera comprises an automatic focusing apparatus for automatically focusing on a subject. Some of the above automatic focusing apparatuses focus on the subject by using a signal obtained from an imaging device. Such automatic focusing method using the video signal has many advantages, for example correct focusing is performed in spite of a depth of field or the like or its structure is very simple because a sensor especially for automatic focusing is not necessary. A hill-climbing servo mechanism which is an example of that automatic focusing method is described in "T.V. Camera Automatic Focal Point Adjustment by Hill-climbing Servo Mechanism", Vol. 17, No. 1 (Whole Number 86), pp.26, 1965, in NHK Technical Report.
FIG. 8 is a schematic block diagram showing a conventional automatic focusing apparatus using the hill-climbing servo mechanism, which is comprised in a video camera.
Referring to FIG. 8, a camera part 1 comprises a focus lens which is moved by operation of the automatic focusing apparatus, a zoom lens for changing an imaging angle and other series of lenses. The focus lens and the zoom lens move in the right and left directions in the FIG. 8 embodiment in accordance with rotary movement of a focus ring 16 provided at an outer periphery of the camera part I. The focus ring 16 is driven to rotate by a focus motor 28. An imaging circuit 2 is provided behind the camera part 1. The imaging circuit 2 comprises an imaging device.
When the subject is imaged, the subject focuses into an image on the imaging device in the imaging circuit 1 by the series of lenses comprising the focus lens in the camera part 1. This image of the subject is converted to a video signal comprising a luminance signal having horizontal/vertical synchronizing signal by the imaging circuit 2 comprising the imaging device. This luminance signal is input to a gate circuit 3 and a synchronization isolating circuit 5. The synchronization isolating circuit 5 isolates vertical and horizontal synchronizing signals from the luminance signals. The isolated horizontal/vertical synchronizing signals are input to a gate control circuit 6 for setting a relatively small prescribed region in the image screen as a luminance signal extracting region (sampling area) for focusing operation.
The gate control circuit 6 comprises a fixed oscillator oscillating with a constant frequency and applies a gate switching signal GC1 which passes only the luminance signal obtained from the above prescribed region to the gate circuit 3 so that the prescribed region may be set as the sampling area in accordance with the vertical synchronizing signal, the horizontal synchronizing signal and an output of the fixed oscillator.
Therefore, the luminance signal corresponding to the sampling area set by the gate control circuit 6 is only applied to an HPF (high-pass filter) 7 provided behind the gate circuit 3. Therefore, a high frequency component is extracted from the luminance signal corresponding to the sampling area which passed through the gate circuit 3 by the HPF 7. Then, amplitude detection of the extracted high frequency component is performed by a detector 8 at next stage. The output from the detector 8, that is, the level of the high frequency component is integrated by an integration circuit every field and applied to an A/D converter 10. The A/D converter 10 converts the output of the integration circuit 9 to a digital value. The digital value is applied to a maximum value memory 11, comparators 12 and 14 and a memory 13 as a value indicating a focused state of the optical system against the subject (a focal point evaluating value).
FIG. 9 is a view showing the relation between the thus obtained focal point evaluating value and a position of the focus lens. In FIG. 9, the abscissa shows a position of the focus ring 16 which indirectly indicates the position of the focus lens and the ordinate shows the focal point evaluating value. For example, in a case where the subject is at a distance of 2m from the camera part 1, the focal point evaluating value indicates the maximum value when the focus lens moved by the focus ring 16 is focused on the subject at a distance of 2m from the camera part 1 as shown in FIG. 9. More specifically, the focal point evaluating value varies while forming a conical shape with the position of the focus lens focused on the subject as its center.
The maximum value memory 11 stores the focal point evaluating value applied from the A/D converter 10 in the first place as the maximum evaluating value until a comparison signal S1 to be described later is applied from the comparator 12 and also inputs the maximum evaluating value to the comparator 12 in response to an input of the focal point evaluating value from the A/D converter 10.
The comparator 12 compares the value applied from the maximum value memory 11, that is, the previous maximum focal point evaluating value with the present focal point evaluating value applied from the A/D comparator 10. Then, the comparator 12 outputs a comparison signal S1 when the present focal point evaluating value is bigger than the value stored in the maximum value memory 11 (first mode) and outputs a comparison signal $2 when the present focal point evaluating value is smaller than the value stored in the maximum memory 11 by a predetermined second threshold value or more (second mode). The comparison signal S1 is applied to the maximum value memory 11 and a motor position memory 17 and the comparison signal $2 is applied to the focus motor control circuit 15.
As described above, the focal point evaluating value is the maximum when the focus lens is focused. Therefore, as the focus lens approaches the focused position from a nonfocused position, the focal point evaluating value is increased. Therefore, the first mode shows the state ,where the focus lens moves toward the focused position and the second mode shows the state where the focus lens passed through the focused position and goes away from the focused position.
The maximum memory 11 restores the newest focal point evaluating value applied from the maximum focal point evaluating value A/D converter 10 in response to the output S1 of the comparator 12, that is, in the first mode in which the focus lens has not yet reached the focused position. Therefore, the maximum value of the focal point evaluating values up to the present is stored as the maximum value evaluating value in the maximum value memory 11.
On the other hand, the focus motor control circuit 15 drives the focus motor 28 so as to move the focus lens backward or forward at the same time when the camera starts to image the subject. Then, the focus motor control circuit 15 reverses the rotating direction of the focus motor 28 in the second mode in which the focus lens passed through the focused position in response to the comparison signal S2 output from the comparators 12 and also in a third mode in which the moving direction of the focus lens is not appropriate (gone away from the focused position) in the initial stage in response to a control signal S3 from a comparator 14 to be described later. Thus, the focus lens changes its moving direction from a direction in which it approaches the imaging device to a direction in which it goes away from it or other way around, with the result that the focus lens starts to move toward the focused position again.
Referring to FIG. 9, for example, in a case where the subject which is at a distance of 2m from the camera part 1 is imaged, when the focus lens starts to move from a position P which is at a distance of 10m from the camera part 1 and is focused on the subject in a direction (a) in which the focus lens approaches the focused position, the focal point evaluating value starts to monotonously increase until the focus lens reaches the focused position Q. Therefore, the comparison signal S1 is output from the comparator 12 and then the contents of the maximum value memory 11 and the motor position memory 17 are continuously renewed until the focus lens reaches the focused position. At this time, the focus motor is controlled by the focus motor control circuit 15 to successively move the focus ring 16 so that the focus lens approaches the focused position. Thus, the focus lens reaches the focused position and passes through this. In this case, if a first threshold value is a reverse reference value with a width shown by A in FIG. 9, when the focus ring 16 reaches a position R1 shown in FIG. 9 after the focus lens passed through the focused position, the comparison signal S2 indicating the second mode is output from the comparator 12. In response to this the focus motor 28 is controlled by the focus motor control circuit 15 and then moves the focus ring 16 in the reverse direction, whereby the focus lens starts to move toward the focused position again.
Meanwhile, the motor position memory 17 is provided to store a focus lens position X which is the nearest to the focused position until the present time. The motor position memory 17 receives an output from a focus ring position detecting sensor 29 connected to the focus ring 16. The focus ring position detecting sensor 29 detects the position of the focus ring 16 to indirectly detect the position of the focus lens and then outputs a focus lens position signal indicating the position of the focus lens. Then, the motor position memory 17 renews its contents, that is, the focus lens position X so that the present focus lens position signal from the focus lens position detecting circuit 29 is stored therein in response to the comparison signal S1 output from the comparator 12. More specifically, in the first mode the contents of the motor position memory 17 always corresponds to the present focus lens position.
However, in the second mode, since the comparison signal S1 is not applied to the motor position memory 17, the motor position memory 17 continues to hold the contents stored when the first mode is changed to the second mode, that is, holds the focus ring position signal indicating the focus lens position (focused position) in which the focal point evaluating value is the maximum. More specifically, the focus lens position corresponding to the focal evaluating value which is stored in the maximum value memory 11 as the maximum evaluating value is always stored in the motor position memory 17.
Meanwhile, a comparator 18 compares the contents of the motor position memory 17 with the focus ring position signal from the focus ring position detecting sensor 29 and when they coincide with each other, it outputs a prescribed control signal CL to the focus motor control circuit 15. The focus motor control circuit 15 stops the focus motor 28 in response to this control signal CL. Accordingly, the movement of the focus ring 16, that is, the movement of focus lens is stopped. At this time, the contents of the motor position memory 17 in the second mode is the signal indicating the focus ring position in which the focal point evaluating value is the maximum. Therefore, in the second mode, the focus motor 28 starts to rotate in the reverse direction in response to the output signal S2 from the comparator 12 and when the focus lens reaches the focused position again, the comparator 18 operates as described above and then the focus motor stops and the focus lens stops at the focused position.
Thus, basic automatic focusing operation of this apparatus in which the prescribed region in the imaging screen is set as a focus detecting region is completed. As can be seen from the above, a focal point evaluating value forming part 30 for finding the focal point evaluating value in the focus detecting region comprises the HPF 7, the detector 8, the integration circuit 9 and the A/D converter 10, a focal point evaluating value change detecting part 31 for detecting a change of the focal point evaluating value in accordance with movement of the focus lens comprises the maximum value memory 11 and the comparator 12, and a focused position detecting part 32 for detecting that the focus lens reaches a wrong focused position after the focus motor 28 rotates in the reverse direction comprises the motor position memory 17 and the comparator 18. In addition, a cut-off frequency of the HPF 7 is generally selected from 200 kHz to 800 kHz or more.
Meanwhile, the focus motor control circuit 15 stops the focus motor 28 and outputs a lens stopping signal LS to the memory 13 at the same time in response to the control signal CL from the comparator 18.
The memory 13 keeps the focal point evaluating value input from the A/D converter 10 in response to the lens stopping signal LS output when the automatic focusing operation is completed, that is, the focal point evaluating value when the subject is in focus until the next lens stopping signal is applied and then outputs it to the comparator 14 of the next stage. Thus, the present focal point evaluating value is applied from the A/D converter 10 to the comparator 14. After the automatic focusing operation is completed, the comparator 14 compares the output of the memory 13, that is, the focal point evaluating value when the subject is in focus with the present focal point evaluating value. Then, when the difference between the present focal point evaluating value and the contents of the memory 13 is a prescribed second threshold value or less, the comparator 14 outputs a subject change signal S4 indicating that the subject is changed to the focus motor control circuit 15. The focus motor control circuit 15 drives the focus motor 28 in either direction and starts the above series of focusing operation again in response to the subject change signal S4. As a result, the automatic focusing operation is performed following the change of the subject. Thus, a subject change detecting part 33 for detecting the change of the subject comprises the memory 13 and the comparator 14.
At the same time, the subject change detecting part 33 has the function of determining whether the rotating direction of the focus motor 28 is appropriate or not just after the automatic focusing operation starts, and correcting it to the right direction. More specifically, the memory 13 stores not only the focal point evaluating value when the automatic focusing operation is completed but also the focal point evaluating value applied from the A/D converter 10 when the automatic focusing operation starts. Then, the comparator 14 outputs the control signal S3 to the focus motor control circuit 15 when the focal point evaluating value applied from the A/D converter 10 just after the automatic focusing operation starts is less than the focal point evaluating value (referred to as an initial value hereinafter) stored in the memory 13 when the automatic focusing operation starts (when the present lens moving direction is not directed to the focused position). The focus motor control circuit 15 reverses the focus motor 28 in response to the control signal S3 from the comparator 14. At the same time, the control signal S3 is input to the comparator 18 and inactivates comparison operation of the comparator 14. The comparator 14 continuously outputs the control signal S3 until the present focal point evaluating value becomes more than the initial value stored in the memory 13. As a result, the control signal CL is not output from the comparator 18 until the present focal point evaluating value becomes more than the focal point evaluating value when the automatic focusing operation starts and the focus motor 28 continuously moves in the direction opposite to the first driving direction. Therefore, even if the moving direction of the focus lens when the automatic focusing operation starts is not appropriate, the focus lens is surely moved in a direction in which the focal point evaluating value becomes the maximum.
Referring to FIG. 9, for example, when the subject at a distance of 2m from the camera part 1 is imaged, if the focus lens starts to move from a position P which is at a distance of 10m from the camera part 1 and is focused on the subject in a direction in which it goes away from the focused position (shown by (b) in FIG. 9), the focal point evaluating value is decreased as the focus ring 16 moves. At this time, if a second threshold value is a reverse reference value with a width shown by B in FIG. 9, when the focus ring 16 reaches a position R2 in FIG. 9 after the automatic focusing operation starts, the control signal S3 is output from the comparator 14. Accordingly, the focus motor 28 starts to move the focus ring 16 in the reverse direction. Then, since the operation of the comparator 14 is not activated until the focus ring 16 passes through the position P in FIG. 9, the focus ring 16 returns the focus lens to the position when the automatic focusing operation starts and then continuously moves it toward the focused position. Thereafter, when the focus ring 16 moves slightly beyond a focused position Q, it returns to the focused position and then stops there by the above operation of the motor position memory 17 and the comparator 18 or the like.
As described above, the focal point evaluating value forming part 30, the focal point evaluating value change detecting part 31, the focused position detecting part 32 and the subject change detecting part 33 control the focus motor control circuit 15, whereby the automatic focusing operation utilizing hill-climbing servo mechanism is performed.
In addition, not the whole screen but the prescribed region at the center of the screen is generally the object of focusing (this region is referred to as an on-focus detecting region) in order to focus into the image projected in the center of the screen.
FIG. 10 is a view showing a structure of an automatic focusing apparatus shown in which a zoom motor 35 and a zoom switch 36 for changing an imaging angle are further provided in the automatic focusing apparatus shown in FIG. 8.
The zoom lens is driven by the zoom motor 35 and moves in the camera part 1 in parallel with an optical axis. Thus, a focal distance of the lens system in the camera part 1 varies, whereby the imaging angle varies. The zoom motor 35 moves the zoom lens in response to a key input from the external switch (zoom switch) 36 which is provided so that the user can vary the imaging angle in accordance with necessity. More specifically, when the zoom switch 36 is pushed, the zoom motor 35 operates to move the zoom lens in a direction in which the imaging angle is increased (wide angle side) or in a direction in which it is decreased (telephoto side). Therefore, while the zoom switch 36 is pushed, the image taken by the imaging circuit 2 is expanding or reducing.
As described above, the conventional automatic focusing apparatus using the hill-climbing servo mechanism performs focusing operation by determining whether the subject is in focus or out of focus in accordance with an added value with an amplitude of a prescribed high frequency component comprised in a luminance signal for one field obtained by imaging the subject.
However, a frequency band of the obtained luminance signal or its level varies with brightness or contrast of the subject. Therefore, in some cases there is no frequency band to be extracted in the obtained luminance signal or the level thereof is very low even if it exists according to the condition of the subject. Thus, in some cases the prescribed high frequency component can not be obtained or the level thereof is very low even if it is obtained according to the subject. In these cases, the focal point evaluating value can not be obtained or the value is not correct even if it is obtained. Therefore, it is not possible to focus on such subject in accordance with the prescribed high frequency component in view of its principle.
Meanwhile, since the conventional automatic focusing apparatus using the hill-climbing servo mechanism has no function of determining a spectrum of a video signal of the subject at all, the above series of focusing operation is performed regardless of the difference in spectrum of the video signal of the subject. More specifically, the conventional automatic focusing apparatus performs focusing operation both on a subject in which the luminance signal comprising no prescribed high frequency component is obtained and on a subject in which the prescribed high frequency component is sufficiently obtained. Therefore, according to the conventional automatic focusing apparatus using the hill-climbing servo mechanism, when the subject in which the luminance signal comprising no prescribed high frequency component is obtained is imaged, the focus lens wastefully continues to operate while the focus lens position in which the focal point evaluating value is the maximum is not found, that is, while the image is out of focus. In addition, although the prescribed high frequency component for finding the focal point evaluating value is derived from the luminance signal obtained from a subject having clear contrast, it can hardly be derived from the luminance signal obtained from a subject having very small contrast such as a remote mountain or the sky, or a subject having no contrast such as a wall or a ceiling. Therefore, the above problems are generated when a subject having small contrast is imaged.
As described above, since the conventional automatic focusing apparatus has no function of determining the spectrum of the image signal of the subject, the existence of the contrast of the subject can not be detected, so that it does not comprise means for showing the user that the subject imaged at that time can not be in focus from the viewpoint of its principle, that is, that the subject has no contrast. Therefore, even if the above phenomenon occurred, the user can not find its cause and means for changing the subject to that having clear contrast or the like so as to prevent the focus lens from wastefully being moved. As a result, when the subject having no contrast is imaged, the image is out of focus for a long time.
In order to solve the above problems caused by unnecessary drive of the focus lens which can not bring into focus, the contrast detecting apparatus capable of detecting whether the contrast of the subject exists or not is necessary.